This application claims priority of Japanese Patent Application number 2003-068860, filed on Mar. 5, 2003.
1. Field of the Invention
The present invention relates to a radar apparatus equipped with an abnormality detection function, and more particularly to an FM-CW radar apparatus equipped with a function for detecting an abnormality in the frequency modulation width.
2. Description of the Related Art
An FM-CW radar radiates forward a transmit wave frequency-modulated by a modulating signal of a triangular waveform alternating cyclically between an upsweep section and a downsweep section, and produces a beat signal by mixing the wave reflected from a target with a portion of the transmitted wave. When the frequency of the beat signal in the downsweep section is denoted by fb(down) and the frequency of the beat signal in the upsweep section by fb(up), the beat frequency fr due to the distance to the target and the beat frequency fd due to the relative velocity of the target are respectively calculated as
fr=(fb(down)+fb(up))/2xe2x80x83xe2x80x83(1)
fd=(fb(down)xe2x88x92fb(up))/2xe2x80x83xe2x80x83(2)
From these, the distance R to the target and the relative velocity V of the target can be respectively calculated as
R=cxc2x7frxc2x7T/4xcex94Fxe2x80x83xe2x80x83(3)
V=cxc2x7fdxc2x7/2f0xe2x80x83xe2x80x83(4)
(where c is the velocity of light, T is the period of the triangular wave, xcex94F is the frequency modulation width (frequency shift width), and f0 is the center frequency.) Therefore, the values of fb(down) and fb(up) associated with each target are determined from the peaks appearing in the beat signal spectrum in the frequency domain obtained by Fourier transforming the beat signal, and the distance and the relative velocity of the target are determined by calculating the equations (1) to (4).
Here, the equation (3) can be rewritten as
fr=4xcex94Fxc2x7R/Cxc2x7T
Therefore, if the modulation width xcex94F decreases, for example, to 50% for some reason, such as a failure, fr also decreases to 50% of the normal value; accordingly, the distance R calculated from the equation (3) also decreases to 50% of the actual distance, resulting in the generation of a false distance.
A possible method for detecting such a failure that causes xcex94F to change would be to directly monitor the transmission frequency by using a frequency dividing circuit and a counter.
This method, however, would require that a circuit for monitoring the transmission frequency be added in the FM-CW radar construction, and would thus lead to a substantial increase in cost.
It is, accordingly, an object of the present invention to provide a radar apparatus equipped with a function for detecting an abnormality in the frequency modulation width at low cost.
A radar apparatus equipped with an abnormality detection function according to the present invention comprises: a distance measuring unit measuring a distance to a target; relative velocity measuring unit measuring a relative velocity with respect to the target; and a unit detecting a discrepancy between the measured distance and the measured relative velocity, based on the distance to the target measured by the distance measuring unit, the relative velocity measured by the relative velocity measuring unit with respect to the same target, and the elapsed time.